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Response of Medjool Dates
to Boron under Conditions of Salinity
Introduction
Boron is an important element in association with plant development and agricultural production. Boron in some soils and irrigation waters reaches concentrations that are found to be toxic to plants. Commonly, boron toxicity occurs in arid regions where irrigation water contains high boron. These regions are often also associated with high water demands for plants and with high salinity in soils and irrigation water. In the Southern Arava Valley, date plantations are traditionally irrigated with saline water. Today, many orchards have switched to effluent water. Effluent from municipal sources in arid regions is commonly both saline and high in boron content. Municipal effluent from the city of Eilat is particularly high in boron. This is due to much of the city’s water supply originating as sea water and that the current method for desalination used leaves high levels of boron.
Date
palms are regarded as tolerant to salinity (Fur and Armstrong 1962,
Maas 1984) Maas reports threshold value of 4dS.m-1
for onset of yield decrease. Later works (Aljaburi 1997, Hassan 1990,1993)
show that date palms are salinity tolerant due to osmotic compensation
brought about by concentration of sodium and calcium in the leaves.
Meiri (1997, 1993) found that increase in irrigation water salinity
resulted in decreased vegetative growth rate. Several of the researchers
document an increase in fruit quality with increased irrigation water
salinity. In general, there is very little literature regarding boron
in date production. The USDA salinity laboratory in Riverside California
has classified date palms as very tolerant. This classification is based
on observations and not on controlled experimental work. There is no
documentation of response to high salinity and high boron together and
therefore it is not known whether cumulated or additive effects are
expected under such conditions or if one of the stress causing factors
will be dominant.
Field experiments attempting to address questions of irrigation quantity and quality in date production are complicated and expensive for several reasons. First, the root system of the date is large and extensive; diameter of the area containing roots can reach 10-15 meters per tree. Therefore, separation of treatments demands extremely large areas and plots. Second, the date has a “long memory”. Yield response and other physiological responses occur up to a year after stress is experienced. Third, the investment in establishing a new orchard is high and therefore it is not practical or possible to apply treatments which will cause real damage to commercial orchards. Without such treatments, it is impossible to truly learn plant response to salt and boron caused stress.
In order to overcome the stated problems, this project considers the issues from two approaches:
1) Lysimeter study of combined boron – salinity response.
2) Statistical analysis of data collected from commercial date orchards in the Arava Valley irrigated with a variety of water sources.
